The invention relates to a method and apparatus for automatic measurement of impedance and more particularly is directed to a time-lapse based method and apparatus for measuring the impedance of an unknown component.
Those skilled in the art are well aware that the prior art includes many means for measuring the impedance of circuit elements. One of the oldest and most classical methods of impedance measurement involves some form of AC Wheatstone or four-branch bridge circuit within which the unknown component in one of the arms or branches is compared to a standard component in another branch. A balance is obtained by adjusting ratio arms in the bridge until a null voltage is obtained at which time the unknown component reactance can be determined. As long ago as 1898, 27 different bridges were described for measuring impedance and other methods were later described. See H. Rowland, "Electrical Measurement by Alternating Currents", American Journal of Science, 4th series, Vol. 4, pp 429-448 (1897) and Phil. Mag. 5th series, Vol. 45, pp 66-85 (1898).
These older bridge methods all required considerable operator skill, especially when a large resistive component was associated with the reactive component, that is a low quality factor (Q) or a high dissipation factor (D). Difficulties were increased because two adjustments are needed to null the bridge and the two adjustments interact.
Other impedance measuring devices that do not use adjustable elements to obtain a balance are referred to as impedance meters. Some of these either keep the voltage across the unknown impedance constant or the current constant, so that the measured current or voltages are proportional to impedance or admittance with only one quantity requiring measurement. This in effect is division by a constant. Other meter type instruments measure voltage and current and obtain the ratio by analog division methods.
More recent methods employing electronic circuits compare the AC signals simultaneously developed across the series combination of the unknown component and a reference resistor. To obtain the appropriate vector component a phase-sensitive detector is employed, the average output of which is presented to an analog-to-digital converter for further digital processing by a microprocessor. The disadvantage of this method is the necessity for precisely maintaining the phase relationship between the test signal and the phase sensitive detector control signal. This problem has been solved as described in U.S. Pat. No. 4,196,475 to Hall but the described method still requires phase detecting and analog-to-digital conversion circuits which are slow in operation and unnecessarily complex.
The only prior art patent which would appear to have any bearing on the instant invention is U.S. Pat. No. 4,196,475 which utilizes a phase sensitive detector and an analog-to-digital converter and which uses voltage measurements rather than time-based voltage transitions.